Many laser imageable coatings typically include a dye and a developer, where each is disposed in a separate phase. The color is developed through a reaction between the dye and the developer upon melting of the coating and mixing of the phases. For example, a colorless or nearly colorless fluoran leuco-dye can react with an acidic developer. Proton transfer from the developer opens the lactone ring of the leuco-dye resulting in extension of the conjugate double bond system and color formation. These coatings are complex chemical systems. They may provide a good marking contrast and sensitivity but have some deficiencies such as:
a) The process of coating preparation may be quite labor-intensive. Color-forming components (leucodyes, for example) should be present in the coating formulation as finely dispersed phase (=<1 um). Particle size reduction of the color-former is an additional process.
b) To enable better sensitivity, the color-former phase in described above formulations should be sensitized with laser radiation absorber—usually through alloying it with leuco-dye. Preparation of the alloys of leuco-dyes and radiation absorbers is an additional process.
c) In order to enable high marking contrast, the content of the dispersed phase in the described above formulations is very high (15-40 wt. %). As a result, the viscosity of the inks is also very high (typically in the range 100-5000 poise). The high viscosity of the inks limits the types of methods that can be used for their application to a substrate.
Therefore, there is a need in the art for laser imageable coatings that overcome one or more of these disadvantages.